It is a bit difficult to do a step-by-step for modem-routers, as there are so many models and they all operate a bit differently for settings. Many modem-routers have settings that are accessible through a browser. See its manual (it should list the URL or IP). I would suggest to look around its settings, without changing anything, until you find a mention for “security”, “encryption”, or “WPA”.
I could show you my settings screen, but your settings will be differently organized.
![](https://discussions.apple.com/content/attachment/9ed3e6b0-e878-4aaf-92cb-1a61ddb912ae)
If you got your modem via your ISP, then ask them for assistance on these WPA settings.
If you bought the modem through a store, then ask the manufacturer for support.